Over the past several decades, the basic principles which govern the operation of membrane devices have become well known. How to use those principles to maximixe the efficiency of a membrane device, referred to as a "module", is not.
In the particular instance when the membrane is in the form of a capillary tube or hollow fiber and used for filtration, the membrane material divides the module into a "feed zone" and a non-feed zone referred to as a "permeate zone" irrespective of whether the module is used in a filtration application. The feed which is introduced either externally (referred to as "outside-in" flow) or internally ("inside-out" flow) of the fibers, is resolved into "permeate" and "concentrate" streams. Many physical considerations affect the operation of a module. For example, the permeability and rejection properties of the membrane; the process flow conditions such as pressure, rate of flow, temperature, etc.; the physical and chemical properties of the feed and its components; the relative directions of flow of feed and permeate; the thoroughness of contact of the feed with the walls of the fibers; and other parameters, each has a direct effect on the efficiency of the module. The goal is to maximize the efficiency of the module and to do so in a practical and economic manner. Readily apparent is that channelling of the feed due to uneven distribution of the fibers, will result in substantially poorer performance of the module than if the feed flowed evenly over the surface of each fiber in the bundles of fibers. Since the rate of transfer of the desired component of the feed from one side of the membrane to the other is necessarily relatively slow, to obtain an optimum rate of transfer requires maximizing the surface area of the membrane and maintaining an effective driving force such as a concentration or pressure differential between opposed surfaces of the fibrous membrane. As a result it becomes essential to use a multiplicity of long fibers of small diameter within the casing of a module so as to minimize the void (unoccupied space) therewithin without packing the fibers together too tightly, and to use as high a fluid velocity or pressure differential as the physical and economic circumstances will permit. But long fibers are susceptible to damage, the longer the fiber, the greater the susceptibility.
If the fibers are packed too tightly in "outside-in" flow, the throughput of feed cannot be increased in proportion to the number of fibers used because of the increased pressure drop. If the length of fibers is increased too much the resistance of the flow path within the bores of the fibers becomes a limiting factor which limits the throughput of feed, though the pressure drop through the bundle of fibers in the module may not be a factor.
In the particular instance of filtration, using such "outside-in" flow of feed, not only does the feed flow through the path of least resistance, namely the largest voids, but it will also tend to collect in some voids from which flow is impeded. The result is that the concentration of a residual component ("residue") remaining on the outside of the fibers ("concentration polarization"), increases. Some of the residue will pass through the walls of the fibers and increase the concentration of the residue impurity in the permeate. If the residue is a salt, some, when concentrated will precipitate on the fibers' surfaces reducing their effective area available for permeation, a phenomenon known as "fouling".
To counter the problems of poor flow of feed through a module and the resulting inefficient mass transfer of the permeate across the membrane, numerous schemes have been suggested, some more practical than others. All are characterized by a conspicuous absence of details as to packing efficiency of the fibers in the module, and most particularly, how one might "fix" the orientation of the large number of fibers required in a practical module.
Routinely, hollow fibers are "potted" near one, or near opposed ends, in a hardened synthetic resinous "header" which is adapted to be inserted with suitable gasketing means, in fluid-tight engagement with the interior wall of the casing of the module. The end surface of each header appears foraminous because of the planarly disposed exposed ends of the hollow fibers, the solid resin forming a seal around the exterior terminal portions of each fiber.
In addition to coping with the problem of positioning a large number of fibers precisely before they are potted, there are numerous pitfalls in "potting" the terminal portions of fibers in a resin which can be solidified. To begin with, one must find a resin which is sufficiently compatible with the fibers as to form a fluid-tight bond which will survive over the useful life of the module. After having found such a resin one must make sure that movement of the fibers near the resin does not damage the fibers due to the shearing action of the solid resin on their terminal portions, particularly if the pressure differential to be used in the module is substantial. Further, cutting and dressing the solid resin to expose the ends of the fibers may result in plugging many of the fibers.
As if these problems were not enough, one could not avoid having to cope with the geometry of the frame which was to support each arrangement of fibers, whatever it may be, in the bundle to be housed in a module. This required construction of the frame in such a manner as both, to facilitate potting of the terminal portions of the fibers, and also to provide adequate support for the fibers intermediate their potted terminal portions, so the fibers are not damaged by flow of the feed through the module.
We decided to eliminate the frame to avoid dealing with the problems endemic to using one. This approach required that we find some way to pot the terminal portions of the fibers and support the fibers intermediate their ends without using a frame.
We addressed the problem of potting the fibers by dispensing with potting the fibers. Instead, we have substituted a "split-clip" header comprising opposed flexible strips, each with a grid of parallel grooves in which the ends of the fibers are secured when the strips are joined. A "split-clip" header is so termed because when opposing faces of upper and lower sections, at least one of which faces is grooved to snugly embrace a fiber, are joined with the terminal portions of the fibers between them, the joined sections serve to "clip" and mechanically snugly secure the fibers near their ends, between the faces.
The split-clip header performs the same function as the potting resin, except that it avoids the problems of potting; and, using the split-clip header at each end of a fiber assembly, referred to as an "array of fibers" herein (an "array" for brevity), allows precise control not only of the planar orientation of the fibers in substantially parallel spaced apart relationship, but also of the transverse spacing between successive layers of fibers in adjacent fiber arrays in a stack (as will be explained in greater detail hereafter).
We addressed, and solved, the problem of supporting the fibers intermediate their ends, by not supporting them between successive layers with any framework structure. Instead we use relatively short lengths of fibers adapted to withstand the force of feed flowing over them without suffering damage due to shear exerted near the fibers' ends at the inboard face of a split-clip header.
It will be appreciated that, though the description of the invention herein is for "outside-in" flow of feed, the fiber array and a module containing a stack of arrays, may be equally well adapted for "inside-out" flow, for such process considerations demanding such flow.
The module of our invention, the method of constructing it, the frameless fiber assembly we use, the method of constructing such an assembly, and the effectiveness of each of the foregoing in a variety of permeation processes, address the deficiencies of the prior art.